Numerous devices currently exist for measuring the volumetric flow of a moving fluid. Some of these devices employ the Karman vortex-street concept. According to the Karman concept, the erection of an object in a relatively moving fluid creates a fluctuating flow field composed of vortices in the wake of the object. In this regard, the relative movement of the fluid may occur either by passing the object through the fluid or by passing the fluid by an object.
Various attempts have been made to provide a velocity sensor which measures the rate of generation of vortices in the wake of an object. A reliable and relatively simple method and apparatus for velocity sensing which operates on the principle of detecting vortices is disclosed in U.S. Pat. No. 3,680,375 of Joy et al. which is incorporated herein by reference. Basically, the above-cited method determines the rate of generation of vortices by directing a signal through the vortex street at a distance downstream from the object. As the vortices pass through the sonic signal, the pressure differential and mass rotation within each vortex cause an impinging sonic signal to be partially reflected and partially refracted as it passes through the vortex. In other words, when a vortex crosses the signal, energy is scattered by the vortex. Hence, a portion of the directed signal is modulated by a decrease in energy caused by the reflection and refraction. Therefore, measurement of the modulation frequency is a direct measure of the fluid velocity.
Velocity sensors of the vortex-frequency vintage such as the above-referenced patent have been incorporated into flowmeters for measuring a gaseous flow within an enclosure, such as a pipe. In this usage, the velocity sensor is inserted in a body having a known cross-sectional area. When utilizing such a flow meter, the volumetric flow is determined by multiplying the fluid velocity, as measured by the sensor, by the known cross-sectional area of the enclosure.
Although measurement of the volumetric flow of a fluid is useful in numerous fluid mechanical applications, many other applications necessitate the measure of the mass flow of the fluid. The mass flow of a fluid, as distinguished from the volumetric flow, is a product of the volumetric flow, the cross-sectional area of the enclosure, and the density of the fluid medium. Hence, the fluid density must be ascertained.
Although there are many ways to determine fluid density, numerous prior art devices have ascertained fluid density by traversing a sonic signal through the fluid from a signal transmitter to a signal receiver and using the relative energy received as a measure of the fluid density. These devices typically measure the voltage across the sonic signal receiver as an indication of the acoustic impedence of the fluid (which is the product of fluid density and the velocity of sound in the fluid). Prior art devices concerned with the acoustic impedence measurement for density include numerous U.S. patents to Jack Kritz. A non-exclusive citation of mass flowmeter art employing this concept include U.S. Pat. Nos. 3,020,759 and 2,959,054 to Welkowitz; 2,991,650 to Katzenstein; and 3,188,862 to Roth.
In the above regard, no mass flowmeter currently available both determines fluid density by measuring acoustical impedence in the above generally-described manner and determines volumetric flow using the preferred Karman vortex method. Although the possibility of such a flowmeter is alluded to by an applicant herein in an article entitled "Air Flow Measurement for Engine Control" which appeared in the February 1976 issue of the Society of Automotive Engineers Journal (SAE Paper Number 760018), no significant embodiment was known or disclosed at the time of that article.
After determining the density of the fluid, the density is often converted to a standard value. The mathematical conversion to a standard value, such as ambient conditions, requires measurement of the temperature of the fluid.
Existing flowmeters, including those utilizing the vortex-frequency concept, employ additional sensors to determine fluid density and temperature. Hence, in these existing meters designed to measure mass flow, one sensor is used to determine the volumetric flow and additional sensors determine the density and temperature parameters.
Numerous disadvantages occur when using multiple sensors to determine the component parameters of mass flow. One practical handicap is the cumbersome additional circuitry which must be engineered into a flowmeter often expected to operate in an small, enclosed environment.
A second problem associated with the employment of multiple sensors is the interference of one sensor with another. The physical proximity of neighboring sensors can spuriously spawn erroneous sonic and electrical components. To minimize interference, prior art devices commonly separate the sensors physically, meaning that the parameters are detected at different locations along the vortex street. For example, a pressure sensor might be placed upstream or downstream of a velocity sensor. Unfortunately, when this sensor positional discrepancy occurs in transient flow conditions, a pressure differential between the separated sensors is ignored. This practice is especially troublesome in rapidly changing flow environments, such as, for example, intake or exhaust gases of an internal combustion engine. Attempts to compensate for the pressure differentials are generally onerous since pressure variations are proportional to the square of velocity.
Considering further the incorporation of multiple sensors into a mass flowmeter, in some structural configurations the presence of an additional sensor along the vortex street can itself cause flowfield modifications in downstream sensing regions.
An additional problem closely akin to the first two is the added expense of additional sensor hardware. Typically the physical construction, mounting, and protection of the actual sensing element is the most costly aspect of a measurement device. Hence, duplicity of these expenses is not economically desirable.
Therefore, it is an object of this invention to provide a new and improved method and apparatus for sensing the mass flow of a fluid.
An advantage of the illustrated embodiments is the utilization of a single sensor for the measurement of volumetric flow, fluid density, and/or fluid temperature, such measurements being spatially made in the same region.
Another advantage of the structure of the invention is that temperature and density can be averaged over a complete signal path length, thus reducing errors due to thermal layering or other temperature or pressure variations.
Still another advantage of the structure of the invention is the realization of substantial cost savings by using a single sensor to perform measurements heretofore performed by a plurality of sensors.